I would select one or two colours max (complimentary colours are easier to work with) and then combine this with more natural or neutral finishes so either woods, stone, white or grey (cold grey will look generally more contemporary or warm greys will be a bit more comforting)

Speak to a supplier and installer of gas fires (or wood burning stoves). Get them to inspect the chimney. Often old building require the chimney to have a liner to stop fumes getting into living accommodation. This may require scaffolding given the height of your chimneys. Without scaffolding expect this job to be £1500 per chimney. Should you need scaffolding then wait to do this job until you have an additional outside job such as roof repairs.

Great project! Have you considered painting or white washing the brick? Could look great!

I like a bit of both really, our kitchen to hallway is door-less but I have just reinstated the hall to living room door because we have a wood burner and want to keep the heat in. I also like the character added by old doors in period properties. I also really like the idea of double pocket doors for large knock-through type openings but unless they're planned from the start of a build there's not many instances where its easily achievable retrospectively. Slightly off topic but I also find the direction a door opens into a room to be important - for me it should open and effectively 'display' for you the largest aspect of the room if that makes sense. Only exception being the bathroom door for added privacy.